The present invention relates to an automatic tuning method and apparatus of a double conversion tuner, and more particularly, to an automatic tuning method and apparatus of a double conversion tuner in which tuning voltage corresponding to picture/sound carrier wave difference of a channel selected by double conversion tuner is transmitted to a voltage controlled oscillator to thereby be re-tuned so that the input filter inserting loss according to said picture/sound carrier wave is minimized.
Generally, tuning systems within television TV receivers use a number of tuners contained respectively with mixers in order to receive signals within a number of TV frequency bands. For example, a first tuner selects the channel within a VHF-TV frequency band (54 to 88 and 174 to 216 MHz) and a second tuner selects the channel within UHF-TV frequency band (470 to 890 MHz). When the TV receiver is preferred to receive a cable television CATV signal, the tuning system for this should be added with a third tuner and mixed.
A double conversion tuning system for preventing complexity of a number of tuners and expensive cost is described for receiving broadcasting (air isolation) VHF-TV and UHF-TV signals as described in title of "high capacity TV" by D. L. arch on consumer electronic IEEE manual CE-24, 1st edition, February 1978, p 39-46. However, simple and cheap tuning systems for receiveing VHF-TV, UHF-TV and CATV signal is required.
For this purpose, heretofore, as the technical contents of U.S. Pat. No. 4,408,348, it is constituted with control device for generating tuning signal having a magnitude determined by frequency of selected channel; a first filter device for selecting an RF (radio frequency) signal corresponding to the channel selected from a first tuning band (low band) containing a lower frequency part of said first cable band (MB-CATV) and said first broadcasting band (L-VHF) in response to said tuning signal; a second filter device for selecting RF signals corresponding to the channel selected from a second tuning band (high band) containing an upper frequency part of said first cable band (MB-CATV), said second broadcasting band (H-VHF) and at least lower frequency part of said second cable band (SB-CATV) in response to said tuning signal, and a filter selecting device for operating said first filter device when the selected channel is at said first tuning band (low band) and for operating said second filter device when the selected channel is at said second tuning band (high band); and a multiple tuning system produces IF (intermediate frequency) signal from the RF signal in sequence of the first broadcasting band, the first cable band, the second broadcasting band and the second cable band.
Control devices produce tuning signals having a magnitude determined by frequency of a selected channel.
The first filter selects an RF signal corresponding to the channel selected from the lower frequency part of the first cable band and the first tuning band including a first broadcasting band in response to the tuning signal. The second filter selects an RF signal corresponding to the channel selected from the first cable band, the second broadcasting and the second tuning band in response to the tuning signal.
A selecting device operates the first filter when the selected signal is within the first tuning band, and operates the second filter when the selected channel is within the second tuning band. And a single conversion varactor tuner is utilized for utilizing broadcasting spectrum of television, and according to this single conversion varactor tuner, since the tuning voltages applied to varactor diodes of a voltage controlled oscillator (hereinafter, this will be abbreviated as VCO) and the radio frequency (hereinafter, called as RF) tuning circuit are the same, it is possible to tune said single conversion varactor tuner by one tuning voltage. However, double conversion tuners should be utilized in order to utilize the broadcasting spectrum by using signals of more kinds than signals inputted to said single conversion varactor tuner and higher frequency signals, and according to this double conversion tuner, tuning voltages applied to varactor diodes of VCO and RF tuning circuit are respectively different.
Since the VCO and RF tuning circuits of said double conversion tuner are tuned respectively by different frequency, when the tuning voltages are applied, there has been problem that input filter inserting loss becomes larger.